Hydroalumination. mechanisms

The retrosynthetic elimination of olefinic stereocenters (E or Z) was illustrated above by the conversion 147 => 148 under substrate spatial control. It is also possible to remove olefinic stereocenters under transform mechanism control. Examples of such processes are the retrosynthetie generation of acetylenes from olefins by transforms such as trans-hydroalumination (LiAlH4), ci5-hydroboration (R2BH), or ci -carbometallation... 

Based on these observations the authors propose the following mechanism for the nickel-catalyzed hydroalumination (Scheme 2-4) During the catalysis process... 

Although some preliminary studies have been carried out [79], uncertainhes still exist about the mechanism of this transformation. The hydroalumination step was... 

To date, synthetically useful enantioselective hydroalumination is limited to the asymmetric reductive ring-opening reaction of bicycHc ethers. In spite of the fact that further studies are necessary to get a detailed understanding of the reaction mechanism, this reaction provides a new route to various cycloalkenol derivatives, which are useful intermediates in the preparation of biologically active compounds. 

This chapter has been organized into three sections. The first section deals with transition metal-catalyzed hydroboration in organic synthesis and this is divided into three subsections - mechanism, chemoselectivity, and stereoselectivity. The second section deals with the application of transition metal-catalyzed hydroalumination reactions in organic synthesis, and this is also divided into three subsections - mechanism, chemoselectivity, and stereoselectivity. The third section examines the application of both hydroborations and hydroaluminations in total synthesis. 

The mechanism for this nickel-catalyzed enantioselective hydroalumination was initially thought to proceed via a nickel-catalyzed hydroalumination followed by the elimination of the organoaluminum with cleavage of the oxygen bridge. However, this seems unlikely in the light of recent evidence (Scheme 11). 

Hydrido(trialkylsilyl)silyllithiums, preparation, 3, 424 Hydroacylations, olefins, 10, 142 Hydroalkoxylations and etherification, 10, 672 in etherification, 10, 683 Hydroaluminations for C-E bond formation characteristics, 10, 857 chemoselectivity, 10, 859 mechanism, 10, 858 overview, 10, 839-870 stereoselectivity, 10, 861 total synthesis applications, 10, 865 characteristics, 3, 275 process and examples, 9, 268 via Ti(IV) complexes, 4, 658 Hydroaminations actinide-catalyzed, 4, 237 in aminations... 

The mechanism of the second reaction is a trans hydroalumination helped by coordination of the alane to the triple bond and external nucleophilic attack. The regioselectivity of the hydroalumination is again determined by silicon the electrophilic alane attacks the alkyne on the carbon bearing the silyl group (the ipso carbon). 

It was demonstrated in [9] that mechanical activation of a mixture of Ca(OH)2 with AI2O3 (at molar ratio 1 4) in a vibratory mill results in partial interaction with the formation of calcium hydroaluminate 4Ca0-3Al203-H20. The authors [9] showed that mechanochemical synthesis of 3Ca0-Al203 H20 occurs starting from Ca(OH)2+Al(OH)3 and Ca(OH)2+ kaolinite mixtures during their activation at room temperature. 

Lautens has shown that the nickel-catalysed hydroalumination of certain alkenes can also be achieved with high enantioselectivity.The mechanism of these reactions has been foimd to be dependent upon a number of factors, and does not necessarily involve an organoalane intermediate. 

A high selectivity and activity for Z-stilbene was observed in the titanium nitride-catalyzed reduction in solution with hydride NaAlH4 as a reducing agent (Figure 2.7) [56]. Filtration tests, recycling of the catalysts, and NMR data indicated that the reaction mechanism is heterogeneous, with hydroalumination intermediates formed in the course of the reduction. 

The nickel-catalyzed hydroalumination of alkenes has been extensively investigated in a variety of contexts. The interaction of aluminum hydrides with nickel(0) and the mechanism of alkene hydroalumination has been studied in detail by Wilke, Eisch, and Zweifel. The most extensive synthetic applications from Lautens involve the hydroalumination of oxabicyclic alkenes followed by ring opening to produce substituted cyclohexenes and cycloheptenes (Scheme 74). - An as5mimetric variant emplo5dng 2,2 -bis(diphenylphosphino)-l,l -binaphthyl (BINAP) is also quite general for various oxabicyclic starting materials. 

Scheme 75 Mechanism of the Hydroalumination-Ring-Opening Sequencel " ...

Me-C5H4) with two equivalents of R2AI-H (R = Bu, Bu CH2) has been reported to afford the corresponding dialkenylsilanes as a result of hydroalumination. The mixed intermediates resulting from mono hydroalumination have been isolated and characterized. Quantum chemical calculations have been employed to provide a more detailed insight into the reaction mechanism. ... 

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